In operation, track circuit apparatus is often subjected to interference from currents of large amplitude. The present invention enables the probability of error due to such interference to be reduced to an arbitrarily low value.
In railway technology, track circuit apparatus is widely used, and has been in use for a long time, to indicate the absence of a train on a given section of track. The principle of track circuits to divide a railway track into successive sections which are electrically isolated from one another by pairs of isolating joints that ensure electrical discontinuity in each of the two rails. An electrical signal transmitter is connected to the two rails at one end of such a section, and a receiver for receiving said signals after they have travelled through said rails is connected to the same two rails, but at the other end of the section. A train entering the section at the receiver end shortcircuits the signals via its wheels and axles, and this electrical short circuit is detected by the receiver which causes the signalling to change state, eg. by changing a green light to a red light at the beginning of the section, thereby preventing a following train from entering the section. The receiver also detects when the first train leaves the section, and again causes the signalling to change state.
Track circuits generally use either pulse type modulation, or a sinusoidal carrier frequency in conjunction with amplitude or frequency modulation. With pulse type modulation, the track transmitter of a given section applies pulses of one polarity and at a specific recurrence frequency to the track, while the transmitters of the adjacent sections apply pulses of opposite polarity and slightly different recurrence frequency. When a modulated carrier frequency is used, both the carrier frequency and the modulation frequency differ between adjacent sections. With both types of modulation (ie. pulse or carrier), the "train" or "no train" state of the receiver is a function of the amplitude of the signal it detects at the appropriate frequencies and/or polarity for its own section. Thus, with pulse modulation, the receiver switches to a "train"state whenever it detects a missing pulse, a pulse of the wrong polarity, or a pulse of too low amplitude, while with carrier modulation, the receiver switches to the "train" state whenever it detects a loss of carrier, a carrier at too low amplitude, or modulation at the wrong frequency.
Unfortunately, such conventional track circuit modulation systems are not reliable enough to guarantee safety. The ever increasing power of modern traction motors and of auxiliary equipment such as various types of converters (eg. current, voltage, or frequency converters) is giving rise to ever increasing levels of interference currents of ever more complex waveforms. Further, the modulation characteristics of conventional systems are fixed and unchangeable once the system is installed. It is thus clear that an interference signal in the frequency band used by a track circuit and having a waveform similar to that of the signals used is capable of causing a receiver to switch into the "no train present" state, with possible disasterous consequences.
Preferred modulation systems in accordance with the present invention greatly reduce the possibility of this happening. A high level of safety is provided in which the probability of mistaken signal identification is insignificant.